Three primary types of electron microscopes are the scanning electron microscope (SEM), the transmission electron microscope (TEM) and the scanning transmission electron microscope (STEM). In an SEM, a primary electron beam is focused to a fine spot that scans the surface to be observed. Some of the electrons in the primary beam are backscattered when they hit the sample. Also, secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons or backscattered electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary or backscattered electrons detected when the beam impacts a corresponding point on the surface.
Backscattered electrons are typically detected using a solid state detector comprising a p-n junction that conducts electricity when the backscattered electrons create charge carriers within the detector depletion region. The term “solid state detector” as used herein refers to a detector that includes a depletion region to detect incident electrons. Backscattered electrons are sometimes detected using a combination of a scintillator and a photomultiplier tube, the backscattered electrons causing a flash of the scintillator and the signal from the scintillator being amplified by the photomultiplier tube. Solid state detectors are typically preferred over “scintillator-photomultiplier” detectors for backscattered electron imaging because they are more compact, and easier to incorporate inside an SEM specimen chamber. Secondary electrons have much lower energies than backscattered electrons and can therefore be steered by modest electric fields inside the SEM specimen chamber. As such, placement of secondary electron detectors is more flexible than that of backscattered electron detectors, and secondary electrons are typically detected using a scintillator-photomultiplier detector. In some circumstance, solid state detectors are also used for secondary electron imaging. Scintillator-photomultiplier detectors have some advantages over solid state detectors, such as favorable bandwidth and signal-to-noise ratio characteristics. Secondary electrons possess low energies (0 to approximately 50 eV) and must therefore be accelerated to an energy typically in the range of 2 to 20 keV and more typically 5 to 10 keV prior to collection by a solid state detector or a scintillator-photomultiplier detector.
In a TEM, a broad beam of high energy electrons is directed to the sample and the electrons that are transmitted through the sample form an image on a fluorescent screen or a photographic plate. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample. Samples are typically thinned to a thickness of less than 100 nm.
In an STEM, a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample. The scattering of the electron beam at different points on the sample depends on the sample properties, such as the atomic number and thickness. An image is formed with the intensity of each point on the image corresponding to the number of electrons collected as the primary beam impacts a corresponding point on the surface. The image contrast in a STEM depends on detecting only electrons that are transmitted with minimum deflections (referred to as “bright field” detection) or detecting only electrons that are scattered at an angle greater than a specified minimum angle (referred to as “dark field” detection.) The term “unscattered” electrons as used herein means electrons that are scattered at less than a pre-specified angle. If a detector were to detect all transmitted electrons, regardless of their exit angle from the sample with respect to the electron beam axis, each pixel would have similar brightness, and the image contrast would correspond to differences between the energy of electrons transmitted through different regions of the sample. Such “electron energy attenuation” contrast arises from the fact that the efficiency of STEM detectors such as solid state detectors and scintillator-photomultiplier detectors is a function of the transmitted electron energy. However, the energy spectrum of transmitted electrons is typically relatively narrow and the corresponding electron energy attenuation contrast is weak and vastly inferior to bright and dark field image contrast.
Because the transmitted electrons have a high energy, both scintillator-photomultiplier and solid state detectors can be used in a STEM. Solid state electron detectors for use in a STEM can have a center region, which detects unscattered electrons, surrounded by one or more annular regions, which detect scattered electrons. A “dead space” is required between the detector regions, to prevent charges generated in one region from leaking into an adjacent region. Multi-region detectors can be replaced using a single region solid state detector or a single region scintillator-photomultiplier detector by placing an aperture or an electron absorbing material (referred to as a “beam stop”) below the sample to enable bright or dark field electron imaging, respectively. Because the primary beam of a STEM is scanned in the same manner as the beam in an SEM, an SEM can be provided with a detector positioned under the sample to operate as a STEM. Solid state detectors are typically preferred over scintillator-photomultiplier detectors because they are more compact and easier to incorporate inside an SEM specimen chamber.
Most electron microscopes operate in a high vacuum to avoid scattering of the primary electron beam. SEMs that operate with the sample under a relatively high pressure are described for example in U.S. Pat. No. 4,785,182 for “Secondary Electron Detector for Use in a Gaseous Atmosphere.” Such devices are known as environmental scanning electron microscopes or as high pressure scanning electron microscopes (HPSEMs). In the context of this application, “high pressure” means a pressure of greater than about 0.01 Torr. Commercially available systems include the ESEM® high pressure scanning electron microscope from FEI Company, the assignee of the present invention. Because electrons in the primary beam are scattered by gas molecules, an HPSEM uses a pressure limiting aperture (PLA) between the relatively high pressure sample chamber and the electron focusing column to maintain a high vacuum in the electron optical column. The diameter of the PLA is sufficiently small to prevent rapid flow of the gas molecules from the higher pressure sample chamber into the lower pressure focusing column. The majority of the electron beam path is therefore in the low pressure column, and the beam travels only a short distance in the gaseous environment below the PLA.
In an HPSEM, the sample that is to be investigated is placed in an atmosphere of a gas having a pressure typically between 0.01 Torr (1.3 Pa) and 50 Torr (7000 Pa), and more typically between 1 Torr (130 Pa) and 10 Torr (1,300 Pa) whereas in a conventional SEM the sample is located typically in a vacuum of about 10−6 Torr (1.3×10−6 mbar). Unlike a conventional SEM, an HPSEM can readily form electron-optical images of moist or non-conducting samples, such as biological samples, plastics, ceramic materials and glass fibers, which would be difficult to image under the typical vacuum conditions of a conventional SEM. The HPSEM allows samples to be maintained in their natural state, without being subjected to the disadvantageous effects of drying, freezing or vacuum coating, which are normally necessary in studies of such samples using conventional SEMs. The gaseous atmosphere of an HPSEM sample chamber provides inherent charge neutralization, that is, the dissipation of surface charge that accumulates on a non-conductive sample as a result of irradiation. Dissipating surface charge increases resolving power of the microscope.
The gas type used in an HPSEM can be selected so as to enable electron-beam induced chemical processing of the sample. In such processes, electrons dissociate gas molecules adsorbed to the sample surface to produce reactive fragments that give rise to material deposition or volatilization in the vicinity of the electron beam. For example, Adachi et al. [Applied Surface Science 76, 11 (1994)] used CH4 to deposit carbon, Folch et al. [Applied Physics Letters 66, 2080 (1995)] used Au(CH3)2(hexafluoroacetylacetonate) to deposit Au-containing nanocomposites, Ochiai et al., [Journal of Vacuum Science and Technology B 14, 3887 (1996)] used (HFA•Cu•VTMS) to deposit Cu-containing nanocomposites, Li et al. [Applied Physics Letters 93, 023130 (2008)] used WF6 to deposit WO3-containing Nanocomposites, and Toth et al. [Journal of Applied Physics 101, 054309 (2007)] used XeF2 to etch Cr.
Conventional scintillator-photomultipliers and solid state detectors can not be used for secondary electron imaging in a gaseous atmosphere, because the detector bias needed to accelerate the low energy secondary electrons to the energy needed for efficient detector operation will cause dielectric breakdown of the gas. An HPSEM, therefore, uses a different mode of detection, referred to as gas cascade amplification. In gas cascade amplification, a voltage is applied between the sample surface and a metal electrode (“anode”). The liberated secondary electrons are accelerated toward the anode and collide en route with gas molecules in their path. These collisions will result in the liberation of new electrons, referred to as “daughter electrons,” and gaseous ions from the gas molecules. The daughter electrons and the ions will move towards and away from the anode, respectively. In their turn, these newly liberated daughter electrons will again collide with other gas molecules, and so forth, so that a cascade amplification of the secondary electron signal occurs. The gas-cascade-amplified secondary electron imaging signal is detected by measuring a current flow induced in electrodes such as the anode or the specimen holder, or by detecting photons generated by the gas cascade.
Combining a high pressure sample environment with an STEM presents difficulties. Conventional gas cascade amplification as used in a HPSEM will not produce adequate contrast in a STEM. The image contrast of a STEM is derived from separately detecting either transmitted electrons that are scattered by the sample or transmitted electrons that are not scattered by the sample. The gas in a high pressure sample environment process, however, scatters the transmitted electrons, and thereby mixes the scattered and unscattered transmitted electron signals, impeding the formation of a useable image. In addition, gas cascade detectors measure the signal induced in an electrode by the motion of charge in the gas, or luminescence generated by the gas cascade. Both of these signals are omnidirectional and serve to further mix the scattered and unscattered transmitted electron imaging signals, thereby impeding further the formation of a useful image.
Conventional scintillator-photomultiplier detectors are impractical for high pressure STEM because they are bulky and difficult to incorporate below a sample in a HPSEM chamber. Hence, solid state detectors are typically used for transmitted electron imaging in HPSEM systems. However, solid state detectors suffer from a number of problems. When imaging a wet sample, water droplets may fall on the detector below. Common samples include cross-sectioned biological tissue and liquids suspended on perforated membranes, and often contain loose materials such as organic residue and nanoparticles. Such loose materials may be transported by the falling water droplets and may deposit on the detector surface. Because a solid state detector requires electrons to strike the depletion region with sufficient energy to excite the electron-hole pairs, any material on top of the detector reduces the number and energy of electrons reaching the depletion region, causing a loss of efficiency of the detector. For example, with just a micron of residue on the surface of the detector, a 30 kV electron might lose half its energy but still deposit 15 kV into the detector, but a 10 kV electron might lose all of its energy in the residue and not deposit any energy into the detector. Alternatively, a transmitted electron may be backscattered by the residue, and hence not deposit any energy into the depletion region. If a scintillator-photomultiplier detector is used in place of the solid state detector, it will suffer from an analogous problem. A scintillator-photomultiplier detector requires electrons to strike the scintillator with sufficient energy to excite photons, and any material on top of the detector reduces the energy of electrons reaching the scintillator and the number of electrons reaching the scintillator, thereby lowering the number of photons generated per transmitted electron, causing a loss of detector efficiency. Solid state detectors and scintillators are also fragile and difficult to clean.
Moreover, solid state detectors and scintillator-photomultiplier detectors are not compatible with some of the precursors used for electron-beam induced chemical processing. Some such precursors are highly reactive, and some, like XeF2, may spontaneously etch detector components. Even if the detector were not spontaneously etched, the detector surface would be subjected to charged-particle-induced reactions as the sample, because the charged particles that are transmitted tough the sample impact on the detector in the presence of the precursors. This problem is particularly severe in HPSEM systems where the precursor pressure at the detector surface is typically similar or equal to the precursor pressure at the sample surface.
Although solid state detectors are commercially available for converting a SEM to a STEM, such detectors are not practical for use in a high pressure environment.